Correlation between image render cap size and game resolution
Ok. I figured it out. Capsize is limited to the window size of the BGE rasterizer.

I implemented something like:

import Rasterizer

windowSize = (Rasterizer.getWindowWidth(), Rasterizer.getWindowHeight())

...
imageRender.capSize = windowSize


Then I can start a standalone player with my desired resolution of 1600 x 1200 px and get the camera image in that very same resolution.